Implantable medical devices designed for patient pain management, also known as neuromodulation devices, have improved dramatically over the past few decades. These implantable medical devices provide much needed relief for patients who suffer from pain which emanates from the spine region. One of the key components of such devices is the implantable electrical stimulation lead. Some electrical stimulation leads, also referred to as percutaneous stimulation leads, are implanted into the patient by insertion of the electrical lead through the skin, through the ligamentum flavum, and into the epidural space or epidural potential space. The electrical lead can then be run along the spinal cord, over the dura membrane, without puncturing the dura membrane. As such, these leads are required to penetrate deep into narrow openings and passages inside the human body and are intended for long term or chronic use by the patient. Once inserted into the epidural space, the lead is intended to reside in the patient over a period of several years.
It is desirable to design an implantable lead to be as minimally invasive as possible because of the reduced risk of infection and improved recovery time of the patient. It is undesirable for the lead to be removed once inserted into the patient if; for example, the electrical lead becomes defective or inoperable. Removing an electrical lead creates additional stress for the patient and increases the risk of infection both when an electrical stimulation lead is removed as well as when a new electrical stimulation lead is implanted.
One such mode of failure in current implantable electrical stimulation leads is impaired electrical contact. Impaired electrical contact occurs when a section of the lead's polymer body becomes entrapped between the conductor wire of the lead and the outer electrode ring that is wrapped around or surrounds the electrical lead body. Such a problem can occur due to the cold flow of the polymer as it migrates between the conductor wire and the electrode ring. This problem is most concerning for chronic or long term implant electrical leads which can reside in a patient over many years.
Electro-physiological catheter leads are typically designed such that the electrode ring is swaged over a polymer cylindrical lead body and conductive wire. An interference fit between the electrode ring and conductor wire around the polymer lead body is created to produce the electrical contact. Delivery of electrical stimulation to the patient is dependent on the electrical connection between the electrode and the conductive wire, and is contingent on the interference fit of the swaged electrode ring over the conductor wire and the polymer lead body. Over time, the polymer of the electrical lead body can become “sandwiched” between the electrode and the conductor wire, which results in impaired electrical conduction between the electrode ring and conductor wire. As a result, the patient's electrical stimulation degrades or ceases.
This problem of impaired electrical contact between the conductor wire and lead body is a major concern for chronic or long term electrical stimulation leads. The problem of impaired electrical contact has been previously addressed through the use of crimping or welding the electrode ring surrounding the conductor wire of the polymer lead body.
In crimping, the electrode ring is pinched around the conductor wire and polymer lead body. This process requires the use of a metal insert or metal sleeve that is placed around the polymer lead body. The metal insert is used as a mechanical stabilizer for the polymer lead body that provides a solid rigid surface that protects the polymer lumen from deforming. The conductor wire is placed in contact between the outside surface of the metal insert and the inside surface of the electrode ring. The electrode ring and conductor wire assembly is then crimped or swaged around the lead body. In such prior art designs, the force of the crimping damages the polymer lead body; therefore requiring metal inserts to serve as a solid rigid protective barrier to the surface and structure of the polymer lead lumen. Further, the use of a sleeve or core insert in the crimping process adds additional width to the body of the electrical lead which is undesirable because it makes it more difficult for the electrical lead to penetrate deep within narrow passages of the human body. The use of a narrow electrical lead is more minimally invasive and minimizes undesirable stress and trauma to the patient.
The problem of impaired electrical contact between the conductor wire and lead body has also been previously addressed through the use of welding. Welding does not require additional crimping components such as a sleeve or core insert, but limits the use of available materials due to its excessive heat and undesirable material reactions that are inherit in the welding process. For example, welding will expose silver in silver cored wire, a commonly used material for use as a conductor; the exposed silver dissolves in the body which could result in bodily harm. Material migration into the body can further result in a weakened bond that increases the probability of a break in the conductor wire, resulting in impaired stimulation lead operation. If a break occurs, the electrical stimulation lead will need to be replaced; therefore requiring the patient to undergo an additional operation creating more unnecessary trauma and stress to the patient.
Accordingly, what is needed is a new chronic electrical stimulation lead that improves upon the previous crimping designs. The new electrical stimulation lead must also exhibit improved electrical contact stability without the material limitations of welding inherent in relatively narrow lead.